Molds with bound molding sand are either manufactured with molding flasks, which consist of two molding-flask halves, whereby this molding flask is as a rule constructed frame-shaped and has running boards on its front and rear sides, with which the molding flask can be transported on a roller train of an automatic mold system. This molding flask provides the sand mold bound therein the necessary strength so that same can be easily transported. The sand for the mold can be clay-bound, which guarantees an inexpensive reprocessing of the molding sand.
Molds without flasks are also known and are assembled dependent on the casting out of a plurality of individual parts, whereby these molds are no longer clay-bound but are chemically bound in order to obtain a sufficient strength for their handling and casting ability. These chemically bound sand molds require expensive machines for the manufacture of the partial molds and cores and machines for, in turn, assembling these parts to form a mold. Aside of the high input of machines, which must here take place, a further disadvantage is that the reprocessing of the sand is difficult. This represents a high expense factor in addition to the expensive molding and mounting machines.
Green-sand molds without a flask are also known and are built of individual identical square-shaped blocks, which have on two oppositely lying sides each defining a portion of the mold cavity. The casting inlet and also the feeder is hereby formed in the blocks. These blocks are then moved towards one another so that their mold cavity portions supplement and support one another. The molding sand is thereby shot in from above, which means that the mold areas, which lie in the shape of the model, do not solidify like the remaining areas. Furthermore the feeder can only exist to a limited degree in the mold and also essentially only laterally of the actual mold cavity, which strongly limits the quality of the feeder. By shooting in the molding sand moreover also a large wear on molds occurs, which makes the process expensive. A further disadvantage is that the cores must be inserted into a laterally directed mold and this must be done in the available cycle time, which on the one hand demands expensive automatic machines and on the other hand is dangerous manual work. An important disadvantage is furthermore that such green-sand molds without a flask are not suited for the manufacture of complicated castings, as for example engine blocks or cylinder heads, in particular when these consist of aluminum.
Moreover, molding sand in molding flasks can not only be clay-bound but is also significantly less expensive to manufacture. In order to manufacture such a molding flask one must merely place each one half of the molding flask separately onto a model plate, whereby the molding flask is then filled with sand. After mechanically compressing the molding sand, the mold half is finished. The horizontally lying mold half is then turned upwardly with the mold cavity therein so that cores can be inserted. The molding flasks are placed one on top of the other for casting, whereby earlier the mold opening and the feeder must be worked into the molding sand in the then upper molding-flask half. This is done prior to the turning of the upper molding flask by using rotating tools. These molding flasks are then cast in a lying position, whereby it is here disadvantageous that only a relatively small volume remains for the feeder in the mold and in addition the feeding of the mold parts lying at the mold bottom is not always optimally achieved.
Whereas it is no problem in the chemically bound molds without a flask to form the feeder and also the pouring funnel right away in the mold, whereby the feeder and pouring funnel are arranged on the side of the mold, which is the most advantageous position for casting of the casted part. The feeding of the solidifying casting is thus significantly more advantageous in chemically bound molds without a flask compared with molds with molding flasks. In particular, it is not necessary to additionally mount yet another feeder onto the molding flask in order to produce the hydrostatic pressure required for the feeding. Of course, these molds are not only expensive with respect to material and manufacture but also demand extremely high installation capital.
It is indeed possible in the case of manufacturing the green-sand molds without a molding flask to at the same time manufacture the pouring funnel without any relieve problems, however, the construction of the feeder creates problems especially since the feeder reduces the cross section available for shooting in the sand. Such molds are less suited for the pouring in of aluminum especially since here large effective feeders are needed.
The core ventilation represents a significant problem both in the case of molds without flasks and in the case of molds with molding flasks, which problem has up to now not been satisfactorily solved.